# Can a 100hp motor drive 40hp loads?

Service factor is well defined by NEMA MG-1, sections of which are easily referred to. While it has been used as a marketing tool in the 440 NEMA frames and smaller range, It is not purely a marketing tool, it is by definition, the difference between temperature rise against name plate conditions, and the maximum allowable temperature rise against the name plate ambient temperature without exceeding thermal limits of the motor's insulation system.

In an attempt to directly answer the question, here are some mathematical considerations.

For 3-Phase AC machines

Horsepower = hp, Motor Current (amps) - I, Voltage = E, Efficiency = EFF, and Power Factor = PF

I = (746 * hp) / (sqrt3 * E * EFF * PF)

The following numbers are taken from typical NEMA motor performance data from a manufacturer of both IEC and NEMA framed motors doing business globally and I must believe they will be fairly typical across standard and brand. I am going to run the calculations for 3/60/460V, however, the results are relative for 3/50/400V and other voltage and frequency combinations.

40HP, 1800 RPM, 324T frame, IE3 (NEMA Premium)

Efficiency at 100% load = 94.1%

Power Factory at 100% load = 0.83

Motor amps at 40HP output are about 48 amps.

Actual Temperature Rise of the motor analyzed is 60C at 40HP output.

Motor starting current is 299A (LRA).

60HP, 1800 RPM, 364T frame, IE3 (NEMA Premium)

Efficiency at 67% load = 94.25%

Power Factory at 67% load = 0.78

Motor amps at 40HP output are about 51 amps.

Actual Temperature rise of the motor analyzed is 27C at 40HP output.

Motor starting current is 451A.

100HP, 1800 RPM, 404T frame, IE3 (NEMA Premium)

Efficiency at 40% load = 94.5%

Power Factory at 40% load = 0.65

Motor amps at 40HP output are about 61 amps.

Actual Temperature rise of the motor analyzed is 12C at 40HP output.

Motor starting current is 810A.

The loaded current values can vary within standard tolerances for windage and friction losses. Windage losses are the most significant of the two and are directly related the full load operating speed of the fan on the motor. The 100HP motor will spin near synchronous speed (1800 RPM), while the 40HP motor will operated at full load RPM of around 1775 RPM.

In an attempt to directly answer the question, here are some mathematical considerations.

For 3-Phase AC machines

Horsepower = hp, Motor Current (amps) - I, Voltage = E, Efficiency = EFF, and Power Factor = PF

I = (746 * hp) / (sqrt3 * E * EFF * PF)

The following numbers are taken from typical NEMA motor performance data from a manufacturer of both IEC and NEMA framed motors doing business globally and I must believe they will be fairly typical across standard and brand. I am going to run the calculations for 3/60/460V, however, the results are relative for 3/50/400V and other voltage and frequency combinations.

40HP, 1800 RPM, 324T frame, IE3 (NEMA Premium)

Efficiency at 100% load = 94.1%

Power Factory at 100% load = 0.83

Motor amps at 40HP output are about 48 amps.

Actual Temperature Rise of the motor analyzed is 60C at 40HP output.

Motor starting current is 299A (LRA).

60HP, 1800 RPM, 364T frame, IE3 (NEMA Premium)

Efficiency at 67% load = 94.25%

Power Factory at 67% load = 0.78

Motor amps at 40HP output are about 51 amps.

Actual Temperature rise of the motor analyzed is 27C at 40HP output.

Motor starting current is 451A.

100HP, 1800 RPM, 404T frame, IE3 (NEMA Premium)

Efficiency at 40% load = 94.5%

Power Factory at 40% load = 0.65

Motor amps at 40HP output are about 61 amps.

Actual Temperature rise of the motor analyzed is 12C at 40HP output.

Motor starting current is 810A.

The loaded current values can vary within standard tolerances for windage and friction losses. Windage losses are the most significant of the two and are directly related the full load operating speed of the fan on the motor. The 100HP motor will spin near synchronous speed (1800 RPM), while the 40HP motor will operated at full load RPM of around 1775 RPM.